1. Field of the Invention
The present invention relates to optical communication, and more particularly, to an extendable loop-back passive optical network (PON) in which network components can be added at lower cost.
2. Description of the Related Art
Optical networks for Fiber To The Home (FTTH) services can be classified into an active optical network (AON) and a passive optical network (PON). An AON includes an active device to manage services for subscribers. The active device performs switching or subscriber management at a remote node (RN). The AON is divided into a first section between a service provider and an RN and a second section between the RN and subscriber devices. The AON is used for a Fiber To The Pole (FTTP) service or a Fiber To The Curb (FTTC) service and accommodates a number of subscribers using a multiplexing communication scheme. However, the AON has a disadvantage in that the RN of the AON requires management and a power supply.
A PON is widely used for FTTH services and includes a passive device at an RN for connecting a service provider with optical network terminals (ONTs) within a single network section. Since the PON uses a passive device, additional management and the power supply are not required for managing optical devices. Furthermore, the PON can provide high-speed data services over a relatively large service area.
Examples of the PON include an Ethernet-passive optical network (E-PON) using a time division multiplexing (TDM) communication scheme. The E-PON is an Ethernet based PON used for point-to-multipoint connections, and Institute of Electrical and Electronics Engineers (IEEE) 802.3ah provides complete standards for the E-PON. In the E-PON, a passive splitter of an RN splits an optical core of a service provider so as to distribute the optical core to subscribers, and a passive optical coupler of the RN couples optical cores of subscribers together so as to transmit data from the optical cores of the subscribers to the optical core of the service provider. Therefore, downstream data from the service provider are naturally broadcasted, and a layer 2 of a subscriber node receives its own data selectively from the downstream data by filtering. On the other hand, upstream data from a plurality of subscribers can interfere with each other while the upstream data are coupled at the RN. Thus, transmission timing of upstream data is reported to ONTs so as to prevent the upstream data interference.
In another type of the PON called wavelength division multiplexing-passive optical network (WDM-PON), the wavelength of an optical source is used. In the WDM-PON, a plurality of wavelengths are multiplexed. The WDM-PON can provide a sufficient bandwidth for high quality of services (QoS) by assigning a wavelength to respective subscribers. In the WDM-PON, an optical core of a service provider transmits wavelengths after multiplexing the wavelengths, and an RN receives the multiplexed wavelengths from the optical core of the service provider and demultiplexes the received wavelengths. The demultiplexed wavelengths are delivered to respective subscribers. Upstream data are multiplexed by the RN and transmitted to the service provider, and the service provider demultiplexes the received upstream data. Unlike in the TDM-PON, upstream data from ONTs (subscribers) do not interfere with each other in the WDM-PON. Thus, upstream data can be transmitted at a desired time. However, the WDM-PON is expensive to construct.
The TDM-PON can be efficiently used for point-to-multipoint networking owing to statistic multiplexing of the TDM communication scheme. However, as subscribers demand more bandwidths, the number of subscriber lines connected to an optical core can be reduced from 32 to 8 or less. Moreover, in the extreme case where each subscriber requires a 1-Gbps bandwidth or larger, only one subscriber can be connected to an optical core. To address these problems, optical link speed can be increased by upgrading the TDM-PON. However, in this case, considerable costs are required. That is, all the OLT, ONTs, and RN of the TDM-PON should be replaced to increase an optical link speed of the TDM-PON. Therefore, it is disadvantageous for network evolution.
There is practically no limit to a subscriber bandwidth in the WDM-PON owing to the characteristics of the WDM communication scheme. That is, a 1-Gbps bandwidth can be provided in the WDM-PON without an addition device. However, since the WDM-PON has a static structure, network resources of the WDM-PON are wasted in the current network environment where subscribers do not require a large bandwidth. Furthermore, it is expected that 1-Gbps services are not required for subscribers in the next several years. Accordingly, the TDM-PON is disadvantageous since it is difficult to evolve (upgrade) the TDM-PON, and the WDM-PON is not suitable for the current networking environment.
Therefore, there is a need for an optical network that can efficiently accommodate increasing subscribers without wasting network resources.